Basis set convergence study of the atomization energy, geometry, and anharmonic force field of SO2: The importance of inner polarization functions

The total atomization energies, geometries, and anharmonic force fields of the SO and SO2 molecules have been studied at the augmented coupled cluster CCSD(T) level near the one-particle basis set limit. The effect of core correlation has been accounted for. The addition of high-exponent d and f “inner polarization functions” to the sulfur basis set was found to be essential for obtaining reliable molecular geometries. The differential effect of core correlation on computed properties is in fact much less important. The CCSD(T) one-particle basis set limit for the symmetric stretching frequencies appears to be higher than the exact value, while the antisymmetric stretching frequency benefits from an error cancellation. The basis set extension effects of diffuse functions and inner polarization functions appear to be nearly perfectly additive. Our best computed atomization energies and geometries agree to within 0.1 kcal/mol, 0.0004 Å, and 0.03 degrees with experiment: The errors in the fundamentals of SO2 are +3.9, −0.4, and +0.4 cm−1. The best computed quartic force field for SO2 should provide a good starting point for a new experimental refinement.